Item 1234
DESIGN: Dragonfly ~ Control - Flight -
Directional - by Collective Pitch Differential
Drawing:
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General:
Complexity:
This entails increasing the collective on one rotor, decreasing the collective on the other. In addition, some inward lateral cyclic, must be applied to the rotor that has the collective reduced. This is getting a little complex, from the perspective of the mixer box.
Forward cyclic, on to the rotor that has the collective reduced, should not be required because the sum of the moment about the Y-axis has not been changed
General:
If some lateral cyclic is applied to the rotor with the least collective, to maintain a level attitude there will probably be some cross coupling, in that the craft will want to move forward, very slightly.
If this lateral cyclic was not included, the craft may roll slightly in the direction that the nose is turning. During powered forward flight, this might not be a problem. During autorotation, it will be a problem because of pedal or control-linkage reversal. This problem could be even more serious at flare during an autorotative landing since the consuming of rotor inertial may 'confuse' the two rotors as to which way to yaw the craft.
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Calculations:
Moment from port rotor, from collective only = 375 lbs * 5" = 1,875 lb-in.
Moment from port rotor, from collective only = 175 lbs * 5" = 875 lb-in.
Difference = 1,000 lb-in CW.
Assuming that the average thrust and drag is at 0.75R, then the balancing force must be 1,000 lbs / 77" = 13 lbs.
Assuming that the drag is 10% of the thrust, then;
The CW moment from the port rotor is 37.5lb * 72" = 2,700 lb-in
The CCW moment from the starboard rotor is 17.5lb * 72" = 1,260 lb-in
plus 1.3 * 72" = 94 in-lb; for a total CCW moment of 1,354 lb-in
It appears that this is an excellent way to obtain yaw while under power. In autorotation, God knows what will happen. Perhaps during the final flare the yaw might work properly.
Note that achieving this will necessitate some complexity in the mixer box. It will require;
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The optimal might be a combination of Opposed Longitudinal Cyclic and Collective Pitch Differential. The Kaman Huskie uses both together and it is not ideal. This will make the mixer box even more complex ~ unless the significantly reduced phase lag can be utilized to give both the lateral and the longitudinal differential from the movement of only one of the two cyclic rods.
Note:
For connection of collective to bottom of control stick see;
DESIGN: Dragonfly ~ Control - Flight - Collective - Overview
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Last Revised: August 19, 2003